1. Field of the Invention
The present invention relates to an optical pickup provided in an optical disk recording/reproducing apparatus for effecting the recording and/or reproduction of information with respect to an optical disk such as a compact disk (CD) and a digital versatile disk (DVD), and more particularly to the construction of an optical system in the optical pickup.
2. Description of the Related Art
FIG. 11 is a diagram schematically illustrating the construction of an optical system in a first conventional optical pickup. The optical system of this optical pickup includes a two-wavelength hybrid laser diode 91, a two-wavelength double-sided diffractive element 92, a polarizing beam splitter 93, an optical axis correcting element 94, a photodetector 95, a quarter wavelength plate 96, an upper mirror 97, and the like.
The two-wavelength hybrid laser diode 91 uses two semiconductor chips, is mounted on an unillustrated mounting base, and is adapted to emit a laser beam a for CD and a laser beam b for DVD separately from the respective semiconductor chips. Since the two-wavelength hybrid laser diode 91 is mounted on the mounting base, the positional accuracy between the semiconductor chips is set stringently, but a situation can occur in which if positional offset occurs, the optical axis changes.
The laser beam (the laser beam a for CD or the laser beam b for DVD) emitted from the two-wavelength hybrid laser diode 91 is passed through the two-wavelength double-sided diffractive element 92, the polarizing beam splitter 93, and the quarter wavelength plate 96. The laser beam is then projected upwardly by the upper mirror 97, is passed through an unillustrated objective lens, and is applied to a track on the optical disk. Then, the applied laser beam is reflected from the optical disk, is made incident upon and reflected by the polarizing beam splitter 93 through the upper mirror 97 and the quarter wavelength plate 96, is passed through the optical axis correcting element 94, and is made incident upon the photodetector 95.
The optical axis correcting element 94 corrects the optical axis of the reflected beam from the polarizing beam splitter 93 directed in the direction indicated by the broken line L1, such that the optical axis of the reflected beam is set in the direction indicated by the solid line L2. The reason this optical axis correcting element 94 is required is that since the two-wavelength hybrid laser diode 91 uses two semiconductor chips, if the distance between the semiconductor chips on the mounting base has changed due to an error, the change needs to be corrected. If this optical axis correcting element 94 is used, as the photodetector 95 it is possible to use a general one which is conventionally available. However, this optical axis correcting element 94 is required for the optical system, and one extra element of the optical system is required, leading to a higher cost. The shape of the light-receiving surface of the photodetector 95 becomes as shown in FIG. 12. In FIG. 12, reference numeral 101 denotes a light-receiving portion for receiving the reflected light of a main beam, and reference numerals 102 and 103 denote light-receiving portions for receiving the reflected light of sub-beams.
In addition, if the optical system having the above-described construction is adopted, the light spot of a main beam 111 and light spots of sub-beams 112 and 113 become elliptical, as shown in FIG. 13. To ensure that the main beam 111 does not undergo a positional offset from a track on the optical disk, the optical pickup is provided with a tracking actuator (not shown) for providing control such that the main beam 111 comes to the center of the track by moving the objective lens of the optical pickup in the radial direction of the optical disk. The sub-beams 112 and 113 are provided for detecting whether the main beam 111 has come to the center of the track.
The long-axis direction of the elliptical shape of the light spot of the main beam 111 or each of the light spots of the sub-beams 112 and 113 is the tracking direction. In actuality, however, infinitesimal light is present, and there are cases where a signal from an adjacent track or a nearby track is picked up, so that signal reproduction accuracy becomes inferior.
FIG. 16 is a diagram schematically illustrating the construction of the optical system in a second conventional optical pickup.
The optical system of this optical pickup includes a laser diode 141 for CD, a diffractive element 142 for CD, a laser diode 143 for DVD, a diffractive element 144 for DVD, a beam splitter 145, a beam splitter 146, a quarter wavelength plate 147, an upper mirror 148, a photodetector 149, and the like. The optical pickup having such an optical system is called a bulk-type optical pickup. The shape of the light-receiving surface of the photodetector 149 becomes as shown in FIG. 17. In FIG. 17, reference numeral 151 denotes a light-receiving portion for receiving the reflected light of a main beam, and reference numerals 152 and 153 denote light-receiving portions for receiving the reflected light of sub-beams.
In the optical system of this optical pickup, since the laser diode 141 for CD and the laser diode 143 for DVD are provided separately, the optical axes of the laser beams splitter 145. Hence, the optical path leading from the beam splitter 145 can be used in common for the CD use and the DVD use. Accordingly, in the optical system of this optical pickup, the aforementioned optical axis correcting element becomes unnecessary.
In the optical system of this construction, the laser beam from the laser diode 141 for CD or the laser diode 143 for DVD is incident obliquely with respect to the tracking direction. Consequently, the light spots of a main beam 161 and sub-beams 162 and 163 assume elliptical shapes in which their long axes are inclined by an angle θ with respect to the tracking direction, as shown in FIG. 18. If such a construction is provided, cases where a signal from an adjacent track or a nearby track is mixed in can be decreased, thereby making it possible to reduce noise components of the reproduction signal.
Although the performance can be improved by the optical pickup of this construction, since the laser diode 141 for CD and the laser diode 143 for DVD are provided separately, the number of optical elements increases as compared with the optical system of the above-described first conventional optical pickup. Hence, there is a tendency of increasing the cost correspondingly.
As described above, in the construction of the optical system of the first conventional optical pickup such as one shown in FIG. 11, since the two-wavelength hybrid laser diode 91 uses two semiconductor chips, the optical axis correcting element 94 is required to correct a change in the distance between the semiconductor chips on the mounting base occurring due to an error. Hence, there been a problem in that higher cost is entailed correspondingly. In addition, the long-axis direction of the elliptical shape of the light spot of the main beam 111 or each of the light spots of the sub-beams 112 and 113 is the tracking direction, as shown in FIG. 13. In actuality, however, infinitesimal light is present, and there are cases where a signal from an adjacent track or a nearby track is picked up, so that there is the problem that signal reproduction accuracy becomes inferior.
In addition, in the construction of the optical system of the second conventional optical pickup such as one shown in FIG. 16, since the laser diode 141 for CD and the laser diode 143 for DVD are provided separately, the number of optical elements increases as compared with the optical system of the above-described first conventional optical pickup. Hence, there is the problem that the cost increases correspondingly.
It should be noted that the conventional technique in JP-A-9-326127 in terms of its characteristic basic construction is included of an optical unit for applying a laser beam onto a recording surface of an optical disk having a different reproduction condition by an objective lens and for introducing the laser beam reflected by the recording surface; a polarizing-surface changeover unit for selectively changing over a polarizing direction of the laser beam to a specific direction in correspondence with the reproduction condition; and a polarization selecting unit upon which the light transmitted through the polarizing-surface changeover unit is incident and which polarizes the light in the specific direction so as to shield an outer peripheral side of the laser beam. In addition, as the optical configuration, there is provided an optical pickup using a two-wavelength hybrid laser diode such as the one described above, a bulk-type optical pickup, or an optical pickup using a two-wavelength monolithic laser diode such as the one which will be described later.
However, since the aperture of the objective lens is limited by a liquid-crystal element, the polarizing direction is perpendicular. Further, the polarizing direction is only adjusted to a horizontal direction or a perpendicular direction by adjusting the rotation of the laser diode. Since the laser beam is not made incident obliquely with respect to the tracking direction, a tilted elliptical light spot is not formed. As a result, there are cases where a signal from an adjacent track or a nearby track is mixed, so that noise components are superimposed on the reproduction signal.
In the conventional technique in JP-A-2001-216677, a multiple semiconductor laser (monolithic laser diode) is used. However, since a grating for CD (one-sided grating) and a grating for DVD (one-sided grating) are provided separately, in the same way as the construction of the case in which the aforementioned two-wavelength double-sided diffractive element is used, an optical axis correcting element is required, so that the number of elements of the optical system increases. In addition, although one-sided gratings are used, the photodetector has a light-receiving surface corresponding to the case where the optical axis correcting element is used.
It should be noted that the conventional techniques of neither JP-A-9-326127 nor JP-A-2001-216677 disclose the shape of the light-receiving surface of the photodetector in the construction in the case where the laser beam is made incident obliquely with respect to the tracking direction by using the two-wavelength one-sided diffractive element and without using the optical correcting element.